Manchester United and Liverpool are set to be investigated by the Football Association after a series of flares were released during the Capital One Cup tie at Old Trafford.

Despite a strong police presence at the third-round fixture and the deployment of so-called “Pyrotechnic Detection Dogs” at the turnstiles, at least four flares were released in the enclosure housing the 7,200 visiting contingent – with a lit flare landing on United fans seated beneath those Liverpool supporters in the East Stand after Javier Hernandez scored. Tensions between the sets of fans were also raised when Liverpool supporters held up a series of large white sheets which together spelled out the message “We are the famous Kopites”.

While the FA is likely to seek observations from United ’s safety officer about Wednesday night’s incidents, it is understood Liverpool will be asked for an explanation due to the flares being released from the visitors’ enclosure.

This game is traditionally regarded as one of English football’s most volatile fixtures and last February flares and smoke bombs were thrown on to the pitch when Liverpool met United in an under-21 fixture in St Helens, although Wednesday night’s match passed largely without other incident.

The return of Liverpool forward Luis Suárez following a 10-game suspension for biting Chelsea defender Branislav Ivanovic increased the sensitivies around the occasion, but the Uruguayan was largely ignored by the United supporters, who have previously taunted the forward as a result of an eight-match ban in 2011 for racially abusing defender Patrice Evra.